Kish refers generally to carbon material that collects at the surface of molten iron (hot metal) from a blast furnace after casting. At casting temperatures, the liquid iron is saturated with dissolved carbon. As the iron cools, it becomes supersaturated with carbon. The carbon comes out of the iron solution as flakes of graphite. Graphite is a soft mineral that occurs as thin plates and is composed of pure carbon. The longer the hot metal cools, the greater the yield of graphite flakes.
In BOF (basic oxygen furnace) steelmaking operations, kish is also produced during the steps of reladling, desulphurization of the hot metal, slag skimming, and ladle treatment. Kish produced during these steps is collected as baghouse dust and varies in quantity and composition. At Bethlehem Steel Corporation's Sparrows Point facility in Maryland, for example, the quantity of kish materials collected as baghouse dust is significant. The following Table-A shows the quantity of kish material collected as baghouse dust during each step at the Sparrows Point facility.
TABLE-A Step in BOF Steelmaking Quantity of Kish Dusts BOF Reladling 1,270 tons per year BOF Desulphurization 240 tons per year BOF Slag Skimming 160 tons per year BOF Ladle Treatment 200 tons per year Total 1,870 tons per year
The kish collected at the Sparrows Point facility was analyzed to determine its chemical and physical properties. In addition, the U.S. Bureau of Mines (USBM) studied the possibility of recovering high purity graphite from kish. See e.g., Nicks L. J., et al., Recovering Flake Graphite from Steelmaking Kish, Journal of Mining, June 1995. According to the USBM, the study was motivated, at least in part, because graphite is a strategic mineral for which there is no domestic supply. The kish sample in the USBM study, like the sample collected at the Sparrows Point facility, was analyzed to determine its chemical properties.
The following Table-B lists the results of the chemical and physical property analyses of the samples taken from the Sparrows Point facility, as well as the results of the chemical analysis conducted on the USBM sample. The data in Table-B demonstrates that there is a high degree of variability associated with the samples, particularly with the carbon content of the samples. The carbon content reflects the amount of graphite present in the kish samples. The amount of graphite is sufficient to make it desirable to subject the kish to beneficiation and/or chemical treatment to recover pure graphite for commercial use. Beneficiation and/or chemical treatment, however, require handling of the kish.
TABLE B CHEMICAL AND PHYSICAL ANALYSIS OF STEELMAKING KISH USBM Sparrows Point Kish Samples Constitutent Kish BOF Related Desulphurizer Skimmer % dry basis Sample 1999 1986 1999 1986 1999 Fe.sup.Total 56.2 57.5 51.1 38.4 23.6 48.2 C 15.6 17.7 24.8 38.7 25.1 29.8 S 3.9 0.05 0.04 0.28 1.19 0.05 P na 0.30 na 0.05 0.01 0.05 Zn " na 0.07 0.30 0.37 na Na.sub.2 O " " 0.05 na 0.36 " K.sub.2 O " " 0.05 " 0.35 " SiO.sub.2 9.5 0.90 1.20 1.40 0.97 1.1 CaO 10 na 0.14 na 20.2 na MgO 2.8 " 0.06 " 12.1 " Al.sub.2 O.sub.3 1.5 " 0.19 " 0.30 " Mn na 0.3 0.9 0.30 na 0.3 Physical Properties Size Analysis % Retained +48 Mesh 4.7 9.4 14.7 +65 Mesh 6.9 9.0 8.7 +100 Mesh 11.5 14.3 10.9 +150 Mesh 10.6 9.6 9.1 +200 Mesh 7.9 6.9 5.9 +325 Mesh 9.3 8.7 7.2 +400 Mesh 2.7 2.4 2.4 +500 Mesh 5.0 4.2 4.0 -500 Mesh 41.4 35.5 36.1 Bulk Density, lbs/cf before 56/104 47/102 and after treatment
Even disposal of the kish, recycling or other treatment, requires some handling. The kish dust, however, is difficult to handle. The kish dust is dry and and contains an ultra fine component.
The distribution of particle sizes of the kish dust is shown in Table-B. Notably, 30 to 40 percent of the kish has a particle size less than 25 micron. High-carbon dust with a particle size greater than 10 microns exhibits a tendency to settle to the ground in ambient air. Part of the kish, however, has a particle size smaller than 10 microns. Particle sizes between about 1 and 10 microns exhibit a resistance to settling both in ambient air and water that increases as the particle size decreases. Below a particle size of about 1 micron, a significant amount of the kish dust remains airborne and does not settle. Thus, part of the kish dust is so fine that it tends to remain airborne. This makes it easy for wind to carry the kish dust far from the source of kish, especially during handling where the dust may become agitated during handling and disposal. An area surrounding the source of kish therefore becomes susceptible to contamination, especially if attempts are made to handle or transport the kish for recycling, beneficiation, and/or disposal.
The tendency for the kish to contaminate surrounding areas is exacerbated by its physical properties. The graphite contained in the kish dust exhibits hydrophobic properties. The hydrophobic properties inhibit wetting. When attempts are made to contain the dust by applying water to the dry kish, the kish dust floats on the water. Water application therefore falls well short of alleviating the dustiness and difficulties associated with handling of the kish. It also falls well short of alleviating the problem of environmental contamination.
In view of the hydrophobic properties of the graphite, attempts have been made to apply a surface active agent (i.e., a surfactant) to the kish, in order to provide a wetting agent along with the water. See e.g., U.S. Pat. No. 3,932,596 to Rohatgi, assigned to the assignee hereof. While those attempts were successful to some extent at containing the kish temporarily, eventually the resulting combination of kish, water, and surfactant would dry. When it dried, the kish again became dusty, and was able to contaminate the surrounding area.
The potential for contamination, however, is not the only disadvantage of the prior techniques. The difficulty associated with handling of the kish and its tendency to become airborne during handling has a negative impact on the recovery of commercially valuable material, such as graphite, from the kish. In particular, the loss of kish dust into the air reduces the total amount of material that can be recovered from the kish.
There is consequently a need in the art for a way of agglomerating the kish, so that it can be handled and/or stored with little, if any, of the kish contaminating or becoming suspended in the surrounding air. A need also exists for a kish product that can be transported easily for recycling, beneficiation, and/or disposal, with little or no contamination of the surrounding air. A need also exists for a way of preventing the agglomerated kish from becoming dusty after it dries. Since the high-carbon dust that makes up the kish may include materials that have commercial value, such as graphite, there is a need for a high-carbon composition which is derived from the kish and which can be handled and/or treated to recover such materials.